1. Field of the Invention
The present invention relates to a prism, projection optical system using the prism, and projection display device using the projection optical system.
2. Description of the Background Art
Projectors (projection display devices) are now arousing interest as a large screen display. For instance, CRT projectors using a compact CRT of high definition and high intensity, liquid crystal projectors using a liquid crystal panel, and DMD (Digital Micromirror Device) projectors using a DMD, have been commercialized.
Further, the category called xe2x80x9cdata projectorxe2x80x9d which not only responds to AV sources such as movies and TV programs, but also projects computer images, is extending rapidly in the market. Its noticeable performance improvements have been made for increasing brightness and contrast of projected image planes, as well as resolution, and uniformity in brightness.
Of these, uniformity in brightness is being one of the most basic requirements as the market of data projectors extends. For instance, in liquid crystal projectors, uniform illuminating technologies such as fly eye integrators have been introduced to attain the compatibility with improvement in brightness.
Meanwhile, light valves such as LCDs and DMDs can be broadly divided into two classes: transmission types and reflection types. The former has a feature that an illuminating optical axis for illuminating a light valve and the optical axis of a projection lens can be coaxially disposed with ease.
It is therefore relatively easy to design an illuminating optical system. This is advantageous in ensuring a basic performance of illuminating the light valve uniformly and brightly.
On the other hand, the latter is inherently disadvantageous because it is often difficult to coaxially dispose an illuminating optical axis and the optical axis of a projection lens, which involves configuration of a complex optical system.
One such illuminating optical system of the reflection type light valve is described in detail in A. G. Dewey, xe2x80x9cProjection Systems for Light Valves,xe2x80x9d Proc. SID, vol. 18/2, pp.134-146, 1977.
FIGS. 28 to 31 are schematic diagrams of illuminating optical systems which are proposed as an illuminating system for reflection type light valve, in the above literature.
Conventional Technique I
FIG. 28 illustrates a typical representative of off-axis illuminating optical systems, which is characterized by illuminating a light valve from a direction that departs from its normal.
In FIG. 28, reference numeral 300 denotes a light valve, numeral 800 denotes a light source, numeral 801 denotes a condenser lens, numeral 802 denotes the final lens of a projection lens on the side on which the light valve 300 is disposed, and numeral 810 denotes an image of the light source 800. Numeral 600 denotes the normal of the light valve 300, and numeral 601 denotes an optical axis of the projection lens.
A bundle of illuminating rays indicated schematically by solid lines in FIG. 28 exits from the light source 800, and is then condensed by the condenser lens 801 and enters the light valve 300. This bundle of rays has the smallest diameter immediately before it enters the final lens 802, thereby forming the image 810 of the light source 800.
Accordingly, the projection lens takes an arrangement of a post-stop lens in which the stop is disposed in the vicinity of the final lens 802. The optical axis 601 of the projection lens is parallel to the normal 600 of the light valve 300, however, since these 600 and 601 are not coaxial, the travel direction of the projected light is inclined with respect to the normal 600, as indicated by the dotted arrow.
In addition, it is suited to avoid the physical impingement between the projecting lens and the illuminating optical system or the bundle of illuminating rays, because the diameter of the final lens 802 can be reduced depending on the post-stop type.
Conventional Technique II
FIG. 29 is a schematic diagram of an off-axis illuminating optical system different from that in FIG. 28. The references used for FIG. 28 have been retained for similar parts in FIG. 29, and description thereof is thus omitted.
In the point that the stop of a projecting lens is disposed within a projection lens system, this optical system has a high possibility that it will be configured by one lens type, the design of which is easier than that of the system in FIG. 28.
Thus, the off-axis optical system as disclosed in Conventional Techniques I or II is excellent in minimizing a focal shift in image planes, because even when projection is made in such an attitude of looking up at the screen, the optical system is less susceptible to keystone distortion (trapezoidal distortion) by which an image plane can be distorted in a trapezoid. That is, it can be said that each optical system is suitable for the front projection type projector which has a high necessity for providing a predetermined elevation angle in the projection direction with respect to the axis of the projector (i.e., which is usually the axis along the vertical direction). In contrast, with either of the optical systems, it is inherently difficult to increase uniformity in illumination because the light valve is illuminated obliquely.
Conventional Technique III
The aforesaid literature further proposes the type in which a prism as shown in FIG. 30 is inserted, and the type in which a reflection mirror is disposed within a bundle of rays as shown in FIG. 31. In FIG. 30, reference numeral 803 denotes a prism, numeral 602 denotes an illuminating light ray, numeral 603 denotes a light ray in the direction of the normal of a light valve 300, which ray is diffracted due to insertion of the prism 803, and numeral 604 illustrates a projected light ray schematically.
It is described that this system can control to some extent the angle formed by the illuminating light ray and projected light ray, thereby increasing the degree of freedom of the optical system""s configuration, whereas this system exerts a great influence on the astigmatism and chromatic aberration of the projection lens, resulting in poor practicability.
Conventional Technique IV
FIG. 31 illustrates a self convergent relay optical system in which with a mirror disposed at the position of a stop in a projection lens, an image of a light source to be formed on the stop is reflected by a light valve 300, and the image is formed again in the vicinity of the mirror.
Compared to the three illuminating optical systems in the foregoing Conventional Techniques I to III, this system is superior in the prevention of distortion of projected images and in illumination performance. However, the following drawback is pointed out. Specifically, when an illuminating light ray reflected by the mirror enters the lens 802, a reflected light occurs on the surface of the lens 802, and this reflected light becomes a ghost light and reaches the screen.
Like this example, if the lens 802 is disposed in the vicinity of the reflection type light valve 300, a sufficient consideration should be given to the influence which can be caused by light passing through the lens 802 two times during its going and returning.
Any of the four illuminating optical systems thus discussed briefly in the foregoing Conventional Techniques I to IV, has difficulty in attaining both the uniform illumination to the light valve and the prevention of impingement between the illuminating optical system and projection lens system.
Note that one optical system which overcomes the above-mentioned drawbacks and is suitable for illuminating variable mirror elements, e.g., DMDs, is disclosed in U.S. Pat. No. 5,604,624.
Conventional Technique V
FIG. 32 is a longitudinal section of a conventional reflection-type light valve illuminating optical system disclosed in U.S. Pat. No. 5,604,624.
In FIG. 32, reference numeral 140 denotes a prism, numerals 141 and 142 denote the side surfaces of the prism 140, numeral 143 denotes an entrance surface of the prism 140, numerals 144 and 145 are first and second surfaces disposed within the prism 140, respectively, numeral 30 denotes a variable mirror element as a reflection type light valve, numeral 804 denotes a light source, numerals 210 and 211 denote a bundle of rays which enters the prism 140 from the entrance surface 143, and then exits from the side surface 142 and travels toward the variable mirror element 30.
The side surfaces 141 and 142 are parallel to each other, and the entrance surface 143 is inclined with respect to the plane direction of the side surfaces 141 and 142. The first and second surfaces 144 and 145 provided within the prism 140 are substantially held parallel to each other, and a clearance (air gap) is interposed therebetween.
The first surface 144, second surface 145, side surfaces 141 and 142 and entrance surface 143 are contained in their respective planes vertical to the drawing. Although the prism 140 is regarded as a unitary article, which comprises in combination with two prism pieces, functionally it may be considered as being comprised of two parts, namely, a first part bounded by the side surface 142, entrance surface 143 and first surface 144, and a second part which includes the second surface 145 and side surface 141.
It is also described that the prism 140 may be constructed of any optical quality material. As a particular material, one which employs PMMA is disclosed.
Since the first surface 144 is so disposed as to perform total reflection of both of the bundles of rays 210 and 211 entering the first surface 144, these bundles of rays 210 and 211 exiting from the light source 804 and then entering the prism 140 deflect largely as shown in FIG. 32, and then are guided to the variable mirror element 30.
When from the light source 804, light of which principal rays are substantially parallel to each other (i.e., telecentric light) enters the prism 140, this light enters the variable mirror element 30 while the parallelism of the principal rays is retained by the total reflection action on the first surface 144.
Thereby, the variable mirror element 30 can be illuminated by the bundle of illuminating rays with the luminance distribution of the light source 804 retained substantially. Therefore, it can be said that the optical system of FIG. 32 is highly advantageous for use as a uniform illumination.
In addition, since the bundles of rays reflected by the variable mirror element 30 do not satisfy the total reflection condition on the first surface 144, the bundles of rays pass through the first surface 144 and exit from the side surface 141 via the air gap and second surface 145, thereby to reach the projection lens 500. It is therefore easy to avoid the physical impingement between the illuminating optical system and projection lens system.
Thus, this conventional technique is excellent in the uniform illumination of the light valve (variable mirror element 30), and in avoiding the impingement between the illuminating optical system and projection lens system. However, the first surface 144 causes different actions on the bundle of incident rays from the light source 804 and on the reflected light from the variable mirror element 30. Specifically, the first surface 144 performs total reflection of the bundle of incident rays from the light source 804, and allows the reflected light from the variable mirror element 30 to pass therethrough.
A brief description will now be made of a DMD as a typical variable mirror element, which reflects light in the direction of the normal of the variable mirror element, with respect to the oblique incidence of a bundle of illuminating rays.
The DMD is one in which a plurality of micromirrors having a ten and several xcexcm square are disposed in matrix form on a silicon substrate by employing a semiconductor manufacturing technology, and which is a reflection type light valve that forms a single plane (reflecting surface) by all these micromirrors.
By tilting the respective micromirrors by electric control, the DMD can produce a first reflection state of allowing an incident light to be reflected in the direction of the normal of the DMD, and a second reflection state of allowing an incident light to be reflected in the direction inclined at a predetermined angle to the normal.
Thereby, when viewed from the side thus reflected, an image display is carried out in combination (modulation) of, for example, the first reflection state corresponding to white indication (the reflected light in this case is called xe2x80x9cON lightxe2x80x9d) and the second reflection state corresponding to black indication (this reflected light is called xe2x80x9cOFF lightxe2x80x9d).
Accordingly, the light modulated by the DMD is projected through the projection lens 500 to a screen (not shown) to display the video information.
FIG. 33 is a perspective view illustrating an example of two micromirrors constituting a DMD (which correspond to two pixels). In FIG. 33, reference numerals 310 and 311 denote micromirrors, numeral 312 denotes their base, numeral 610 is the normal of the base 312. There are shown the micromirrors 310 and 311, each being inclined at an angle of +10xc2x0 or xe2x88x9210xc2x0, with respect to the normal 610.
Numeral 611 denotes the normal of the micromirror 310 (In FIG. 33, it is inclined at an angle of +10xc2x0 with respect to the normal 610 of the base 312). Numeral 612 denotes the normal of the micromirror 311 (In FIG. 33, it is inclined at an angle of xe2x88x9210xc2x0 with respect to the normal 610 of the base 312).
The micromirrors 310 and 311, which have on its surface aluminum deposited thereon, function as a square mirror having high reflectance. In such a case as shown in FIG. 33, the inclination of the micromirrors 310 and 311 is each xc2x110xc2x0, with respect to the normal 610. As a result, the micromirror 310 reflects the light entering from the direction inclined at an angle of 20xc2x0 to the normal 610, in the direction along the normal 610 (This corresponds to the first reflection state.). On the other hand, the micromirror 311 reflects the light entering from the direction inclined at an angle of 20xc2x0 to the normal 610, in the direction inclined at an angle of 40xc2x0 to the normal 610 (This corresponds to the second reflection state.).
Note that more details of the operation and others about the above-mentioned DMD are omitted herein because it is described in detail in, e.g., xe2x80x9cDigital Light Processing for High-Brightness, High-Resolution Applicationsxe2x80x9d by Larry J. Hornbeck, SPIE Vol. 3013, pp. 27-40.
In the case of using the above-mentioned DMD, it is required to avoid that any unwanted light caused in the second reflection state is projected on a projected image plane. This is because the contrast of the projected video may be reduced due to a stray light component caused when the OFF light traveling in the direction inclined at an angle of 40xc2x0 to the normal 610 scatters on the optical path in the projection optical system, alternatively, due to ghost light caused when this OFF light travels through (enters) the projection lens.
In this event, for improving the contrast of the projected image plane, the OFF light has to be guided outside the optical path by some means, or absorbed by a shading member.
For instance, in the optical system characterized by the post-stop lens as described with reference to FIG. 28 (Conventional Technique I), the image-sided aperture of the projection lens can be made small. Therefore, this optical system is substantially advantageous because it can prevent the entry of OFF light.
Referring now to FIG. 34, description will be given of the OFF light behavior in the conventional illuminating optical system as described with reference to FIG. 32 (Conventional Technique V). Note that with regard to a variable mirror element 30, the case of using a DMD will be described (hereinafter referred to as xe2x80x9cDMD 30xe2x80x9d).
In FIG. 34, reference numeral 146 denotes a side surface of a prism 140, numeral 40 denotes a cover glass disposed above the DMD 30, and numeral 613 denotes the normal of the DMD 30. Other numerals are common to those used in FIG. 32, and description thereof is thus omitted.
As indicated by arrows in FIG. 34, the OFF light in the above-mentioned second reflection state exits from the DMD 30, and part of the OFF light reaches a side surface 146 of the prism 140, like light rays indicated by ellipse A of solid line in FIG. 34.
Most of the remaining light rays exit from the prism 140 through the side surface 141, and then travel in such directions as indicated by ellipse B of dotted line in FIG. 34. In this event, the OFF light arrived at the side surface 146 can be handled relatively easily by, for example, applying a photo-absorbent to the side surface 146.
With the arrangement shown in FIG. 34, a considerable ratio of the light rays exiting from the side surface 141 seems to enter a projection lens 500. It is uneasy to make a quantitative examination of the ratio of the bundles of rays indicated by the ellipse B of dotted line, to the entire OFF light, because it depends on the thickness of the prism 140 in the direction of the normal 613, namely, the distance between the side surfaces 141 and 142, and on the size of the DMD 30.
On the contrary, the necessity of preventing the entry of OFF light that causes stray light or ghost light, will be further increased by taking into consideration the fact that the intensity (intensity of light) of the OFF light is approximately the same as that of ON light, and the fact that when the projection lens 500 is a telecentric lens useful for increasing the ratio of quantity of light on the periphery of an image plane, the final lens on the DMD side becomes so large that more components of the OFF light are liable to enter this lens (That is, the possibility of entry is increased).
U.S. Pat. No. 5,604,624 also discloses an optical system that aims to avoid the OFF light entering a projection lens. This optical system is illustrated in FIG. 35.
In FIG. 35, reference numeral 1400 denotes a prism made up of three prism pieces, numerals 1401 and 1402 denote a pair of surfaces with an air gap disposed therebetween, which have the same action as the paired surfaces 144 and 145 described with reference to FIG. 32, and numeral 1403 denotes a surface through which OFF light exits. Other numerals are common to those used in FIG. 32, and description thereof is thus omitted.
With the prism 1400 of FIG. 35, a first pair of surfaces 144 and 145 provides selective transmission of the reflected light from the DMD 30 and reflection action of the illuminating light to the DMD 30.
A second pair of surfaces 1401 and 1402 provides selective transmission of ON light and OFF light and reflection action of OFF light. Therefore, the prism 1400 can substantially overcome a contrast reduction due to the OFF light entering the projection lens 500, which has been the drawback inherent in the optical system shown in FIG. 32.
With reference to FIG. 35, it will be noted that in order to realize the above-mentioned behaviors of the respective light rays, the prism 1400 obtained by combining prism pieces integrally, has to take an extremely large shape than the prism 140 of FIG. 32.
When the prism 1400 is viewed from the projection lens 500, the prism 1400 can be considered as a plane parallel plate bounded by the side surfaces 141 and 142. In the designing, the distance between the side surfaces 141 and 142 is the parameter directly related to the degree of difficulty of optical design. Increasing this distance increases the back focal distance of the projection lens 500, thus resulting in a noticeable high degree of difficulty of design.
In addition to this, increasing the size of the prism 1400 increases the weight and volume of the projector or increases the amount of optical material, which can cause practical inconveniences, e.g., the additional cost.
In U.S. Pat. No. 5,604,624 there are no particular description about such practical disadvantages. Hence, there has been a desire for a compact prism and a compact illuminating optical system which are suitable for a reflection type light valve, especially for a variable mirror element such as a DMD.
According to a first aspect of the invention, a prism disposed between an external variable mirror element and an external projection lens having a projection optical axis parallel to a normal of the variable mirror element comprises: a first inner surface comprising a first end spaced a first distance apart from the variable mirror element and a second end, the first inner surface being inclined from the first end to the second end in a direction away from the variable mirror element, the first inner surface being configured to totally reflect a bundle of externally entered rays and to propagate the bundle of externally entered rays thus reflected toward the variable mirror element; and a second inner surface comprising first and second ends spaced second and third distances apart from the variable mirror element, respectively, the second inner surface being configured (i) to transmit the bundle of externally entered rays totally reflected from the first inner surface to propagate the bundle of externally entered rays to the variable mirror element, (ii) to transmit first and second bundles of rays in a first reflection state among bundles of rays reflected from the variable mirror element, and (iii) to totally reflect a bundle of rays in a second reflection state among the bundles of rays reflected from the variable mirror element, the second reflection state being different from the first reflection state, the first end of the first inner surface being located at an upper part than a predetermined location within the second inner surface disposed between the first and second ends of the second inner surface, when viewed from the variable mirror element, the third distance being greater than both of the first distance and the second distance, the first distance being greater than the second distance, and wherein, when the first bundle of rays in the first reflection state transmitted through the second inner surface enters the first inner surface, the first inner surface transmits the first bundle of rays in the first reflection state to propagate the first bundle of rays in the first reflection state toward the projection lens.
According to a second aspect of the invention, the prism of the second aspect further comprises a third inner surface comprising a first end that is opposed to the first end of the first inner surface and spaced a fourth distance apart from the variable mirror element, and a second end that is opposed to the second end of the second inner surface and spaced a fifth distance apart from the variable mirror element, the third inner surface being a surface opposed to the second inner surface and configured to allow the second bundle of rays in the first reflection state incident thereon after having passed through the second inner surface, to pass therethrough and propagate toward the projection lens, the fifth distance being greater than both of the third distance and the fourth distance, and the fourth distance being greater than the first distance.
According to a third aspect of the invention, the prism of the second aspect further comprises: a first outer surface that is disposed at a location opposed to the first inner surface and is an entrance surface which allows the bundle of externally entered rays incident thereon to pass therethrough and propagate through the prism toward the first inner surface; a second outer surface that is disposed at a location opposed to the variable mirror element and the second inner surface, and comprises a first end corresponding to the first end of the second inner surface and a second end disposed below the second end of the second inner surface, the second outer surface serving as an optical action surface which allows the bundle of externally entered rays incident thereon after having passed through the second inner surface, to pass therethrough and travel toward the variable mirror element, and which allows the bundles of rays reflected from the variable mirror element, to enter the prism; and a third outer surface that is substantially parallel to the second outer surface, disposed at a location opposed to the projection lens, and comprises a first end disposed above the second end of the third inner surface and a second end adjacent to the second end of the first inner surface, the third outer surface serving as an exit surface that allows transmission of both the first bundle of rays in the first reflection state incident thereon after having passed through the first inner surface, and the second bundle of rays in the first reflection state incident thereon after having passed through the third inner surface, the second, third and first inner surfaces being disposed between the second and third outer surfaces.
According to a fourth aspect of the invention, the prism of the third aspect further comprises: a fourth outer surface that is disposed at a location opposed to the first outer surface, and comprises a first end connected to the second end of the second outer surface and a second end connected to the second end of the second inner surface; and a shading member that is disposed on the fourth outer surface and absorbs the bundle of rays in the second reflection state rays incident thereon after having been totally reflected from the second inner surface.
According to a fifth aspect of the invention, the prism of the fourth aspect further comprises a fifth outer surface that is flushed with the second outer surface, and comprises a first end adjacent to the first end of the second outer surface and a second end connected to the first outer surface.
According to a sixth aspect of the invention, the prism of the fifth aspect further comprises: a first prism piece comprising the first outer surface, the fifth outer surface and the first inner surface; a second prism piece comprising the second outer surface, the fourth outer surface and the second inner surface; and a third prism piece comprising the third outer surface and the third inner surface, wherein any one of the first, second and third prism pieces is opposed to the other two via at least one air gap, a fourth inner surface bounded by the first inner surface and the fifth outer surface among outer surfaces which the first prism piece has is opposed to the second inner surface of the second prism piece, and a fifth inner surface bounded by the third outer surface and the third inner surface among outer surfaces which the third prism piece has is opposed to the first inner surface of the first prism piece.
According to a seventh aspect of the invention, the prism of the sixth aspect is characterized in that two opposed prism pieces among the first, second and third prism pieces are fixed by a spacer and an adhesive, each having a coefficient of thermal expansion approximately the same as that of the two opposed prism pieces.
According to an eighth aspect of the invention, the prism of the sixth aspect is characterized in that two opposed prism pieces among the first, second and third prism pieces are located opposed to each other, via thin film coatings provided on areas other than a light passage region, on either of opposed surfaces of the two opposed prism pieces.
According to a ninth aspect of the invention, the prism of the sixth aspect is characterized in that the second prism piece comprises: (a) a prism piece comprising the second outer surface, a first surface opposed only to the fourth inner surface in the second inner surface, and a sixth inner surface bounded by the second outer surface and the first surface of the second inner surface; and (b) a prism piece comprising the fourth outer surface, a second surface opposed only to the third inner surface in the second inner surface, and a seventh inner surface that is bounded by the fourth outer surface and the second surface of the second inner surface and is opposed to the sixth inner surface.
According to a tenth aspect of the invention, a prism disposed between an external variable mirror element and an external projection lens comprising a projection optical axis parallel to a normal of the variable mirror element, the prism comprising first, second and third prism pieces, the prism being capable of selectively deflecting a bundle of first reflection state rays and a bundle of second reflection state rays that correspond to a first reflection state and a second reflection state, respectively, which are generated by the variable mirror element and are different with each other, (a) the first prism piece comprising: a first surface allowing transmission of a bundle of externally entered rays; a second surface performing total reflection of the bundle of externally entered rays after having passed through the first surface, and allowing transmission of the bundle of first reflection state rays incident thereon after having passed through the first prism piece; and a third surface allowing transmission of the bundle of externally entered rays incident thereon after having been totally reflected and propagated through the first prism piece, and allowing the bundle of first reflection state rays incident thereon to propagate through the first prism piece, (b) the second prism piece comprising: a fourth surface being opposed to the third surface and allowing the bundle of externally entered rays incident thereon after having passed through the third surface, to pass therethrough and propagate through the second prism piece, and allowing the bundle of first reflection state rays incident thereon after having propagated through the second prism piece to pass therethrough, the fourth surface performing total reflection of the bundle of second reflection state rays incident thereon after having propagated through the second prism piece; and a fifth surface being opposed to the variable mirror element, and allowing the bundle of externally entered rays incident thereon after having propagated through the second prism piece, to pass therethrough and propagate toward the variable mirror element, and allowing both the bundle of first reflection state rays and the bundle of second reflection state rays incident thereon, each having been generated when the bundle of externally entered rays is irradiated to the variable mirror element, to pass therethrough and propagate through the second prism piece, the fifth surface being capable of totally reflecting the bundle of second reflection state rays incident thereon after having been totally reflected on the fourth surface and then propagated through the second prism piece, and (c) the third prism piece comprising: a sixth surface having an end adjacent to an intersection between the second and third surfaces and being opposed to the fourth surface, the sixth surface allowing transmission of the bundle of first reflection state rays incident thereon after having passed through the fourth surface; a seventh surface being opposed to the second surface and intersecting the end of the sixth surface, the seventh surface allowing transmission of the bundle of first reflection state rays incident thereon after having passed through the second surface; and an eighth surface being opposed to the projection lens, and allowing both (i) the bundle of first reflection state rays incident thereon after having passed through the sixth surface and then propagated through the third prism piece and (ii) the bundle of first reflection state rays incident thereon after having passed through the seventh surface and then propagated through the third prism piece, to pass therethrough and propagate toward the projection lens.
According to an eleventh aspect of the invention, the prism of the tenth aspect is characterized in that the second surface is parallel to the seventh surface, the third and sixth surfaces are each parallel to the fourth surface, and an air gap is provided between parallel surfaces.
According to a twelfth aspect of the invention, the prism of the tenth aspect is characterized in that the third surface is flush with the sixth surface.
According to a thirteenth aspect of the invention, the prism of the tenth aspect is characterized in that when xcex1 is an angle formed between the first and second surfaces, xcex3 is an angle formed between the fourth and fifth surfaces, and (xcex2+xcex3) is an angle formed between the second and third surfaces, the angle xcex1 is larger than 38.0xc2x0 and smaller than 50.4xc2x0, the angle xcex2 is larger than 25.0xc2x0 and smaller than 37.4xc2x0, and the angle xcex3 is larger than 16.2xc2x0 and smaller than 24.5xc2x0.
According to a fourteenth aspect of the invention, the prism of the tenth aspect is characterized in that the second prism piece further comprises: a side surface bounded by the fourth and fifth surfaces; and a shading member that is disposed on the side surface and intercepts the bundle of second reflection rays incident on the side surface after having been totally reflected on the fourth surface and then propagated through the second prism piece.
According to a fifteenth aspect of the invention, a projection optical system comprises: a light source; a condensing optical system condensing rays from the light source; a light intensity uniforming element comprising an entrance surface from which light condensed by the condensing optical system enters, and an exit surface from which a bundle of rays having a substantially uniform light intensity distribution exits; a transfer optical system transferring the bundle of rays exiting from the exit surface of the light intensity uniforming element; the prism of the tenth aspect in which the bundle of rays transferred by the transfer optical system enters as a bundle of externally entered rays; a variable mirror element which is disposed at such a location that in the exterior of the prism, its reflecting surface and the exit surface of the light intensity uniforming element are in a conjugate relationship via the transfer optical system and the prism, and which generates, when reflecting a bundle of rays entering from the prism at the reflecting surface, bundles of first reflection state rays and bundles of second reflection state rays which are different from each other in reflection state; and a projection lens receiving the bundles of first reflection state rays that are emitted from the variable mirror element to the prism and then exit from the prism.
According to a sixteenth aspect of the invention, the projection optical system of the fifteenth aspect further comprises a prism holding member that holds the prism by making contact with a portion of the outer shape of the prism except for portions of the prism through which the bundle of rays from the transfer optical system and the bundles of first reflection state rays pass.
According to a seventeenth aspect of the invention, the projection optical system of the sixteenth aspect is characterized in that the prism holding member comprises a portion for intercepting the bundles of second reflection state rays exiting from the prism.
According to an eighteenth aspect of the invention, the projection optical system of the sixteenth aspect is characterized in that the prism holding member comprises a surface opposed to the projection lens, the opposed surface of the prism holding member comprising a light exit opening sized to allow the bundles of first reflection state rays to pass therethrough.
According to a nineteenth aspect of the invention, a projection type display device comprises: the projection optical system of the fifteenth aspect; a signal generating part configured to generate electric signals for driving the variable mirror element to output the electric signals to the variable mirror element; and a screen configured to receive bundles of rays projected from the projection optical system.
According to a twentieth aspect of the invention, a projection optical system comprises: a light source; a condensing optical system condensing rays from the light source; a light intensity uniforming element comprising an entrance surface from which light condensed by the condensing optical system enters, and an exit surface from which a bundle of rays having a substantially uniform light intensity distribution exits; a transfer optical system transferring the bundle of rays exiting from the exit surface of the light intensity uniforming element; the prism of the third aspect in which the bundle of rays transferred by the transfer optical system enters as a bundle of externally entered rays; a variable mirror element which is disposed at such a location that in the exterior of the prism, its reflecting surface and the exit surface of the light intensity uniforming element are in a conjugate relationship via the transfer optical system and the prism, and which generates, when reflecting a bundle of rays entering from the prism at the reflecting surface, bundles of first reflection state rays and bundles of second reflection state rays which are different from each other in reflection state; and a projection lens receiving the bundles of first reflection state rays that are emitted from the variable mirror element to the prism and then exit from the prism.
The prism of the first or second aspect of the invention is more compact than conventional ones.
The prism of the third aspect is capable of reducing the back focal distance of the projection lens disposed outside.
The prism of the sixth aspect is capable of suppressing degradation of resolution based on the relationship between the face-to-face planes of adjacent prism pieces, thereby to realize a reflecting surface using total reflection action.
The prism of the seventh aspect can suppress change in the relative positions of adjacent prism pieces, due to variations in temperature environment, thereby to suppress degradation of performance caused by such change.
The prism of the eighth aspect can securely form an air clearance between prism pieces, thereby to realize the exact air clearance.
The prism of the tenth aspect is more compact than has hitherto been possible.
The prism of the eleventh aspect can suppress degradation of resolution based on the relationship between the face-to-face planes of adjacent prism pieces, thereby to realize a reflecting surface using total reflection action.
The prism of the twelfth aspect can facilitate combination of prism pieces.
The prism of the thirteenth aspect ensures a total reflection on a predetermined plane, in the range of refractive index of practical prism materials.
The prism of the fourteenth aspect can reliably intercept the reflected light in the second reflection state of the variable mirror element.
The fifteenth or twentieth aspect can realize a projection optical system providing high brightness and high contrast.
The seventeenth aspect can realize a projection optical system with high contrast which can hold the prism and has a shading function.
The projection optical system of the eighteenth aspect can suppress degradation of performance due to contamination of the prism.
The nineteenth aspect can realize a projection type display device providing high brightness and high contrast.
It is a major object of the present invention to provide a compact prism, a compact projection optical system, and a compact projection type display device.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.